UK light heavyweight Joshua Buatsi earned a win yet disappointing debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the approaches didn't complement each other, with constant grabbing disrupting the fight.
Two judges scored it 96-94 to the winner, with the third a even score.
"It wasn't my best showing but what defines champions? They succeed," the victor commented in the ringside as he was greeted with negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts thought the challenger, thirty-one, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
The Champion Earns Triumph in Slow Match
His career development has slowed in recent times through management issues and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after suffering a hand injury in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to edge the better rounds.
The audience's tolerance diminished because of constant clinching, with the challenger going down multiple occasions during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester contest to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional tribute before the fight as 10 bells rang in memory of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell disappeared, the stillness became a chant of "there's just one Ricky Hatton".
The fighting within the ring reflected the feeling. Arthur was floored by a significant blow flush on the chin in the early round, standing uncertainly to his footing on the referee's count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, noticeable damage forming under his orbital area as the duo traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, narrow lead and 115-112, but each competitor were determined and courageous, everything their idol Hatton stood for.
